Preheat oven to 350 degrees F (175 degrees C). Grease and flour two 9 inch pans.,______,In a large bowl, mix flour, sugar, and baking soda. Add chocolate mixture, buttermilk, eggs and vanilla. Beat until smooth. Pour batter into two 9 inch round pans.,Bake at 350 degrees F (175 degrees C) for 30 to 35 minutes, or until a toothpick inserted into cake comes out clean.